Time division pulse multiplex system



Eb. 27 1951 w. n. HoUGHToN- 2543338 TIME DIVISION PULSE MULTIPLEX SYSTEM Filed Nov. 15, 1947 9 Sheets-Sheet 2 MSRSQ QQbY INVENTOR D. HOUGHTUN ATTORNEY Feb, 27 1951 win. HoUGHToN TIME DIVISION PULSE MULTIPLEX sYs'rm 9 Smets-Sheet 5 Filed Nov. 15, 191W MAMMA www w .www @www mmm/mmm AT ORNEY Feb, 27, i951 w. D. HoujGHroN TIME DIVISION PULSE MULTIPLEX SYSTEM Filed Nov?. 15, 1947 9 Sheets-Sheet 4 D. HOUGHTON ATORNEY WILLIA iA BY l NNN "www Feb. 27, 1951 w. D. HOUGHTON 2,543,738

TIME DIVISION PULSE MULTIPLEX SYSTEM Filed Nov. 15, 1947 9 SheeS-Sheet 5 ATTORNEY Feb. 27, 1951 w. D. HQUGHTON TIME DIvIsIoN PULSE MULTIPLEX SYSTEM 9 Sheets-Sheet 7 Filed Nov. 15, 1947 @NN in@ MAS# REM. S.

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TIME DIVISION PULSE MULTIPLEX SYSTEM William D. Houghton, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 15, 1947, Serial No. 786,286

2l Claims. l

This invention relates to a multiplex or multichannel system operating on the time division principle, and more particularly, to a pulse multiplex system involving a large number of channels. In a time division multiplex system, a common transmission medium is sequentially assigned to the .diierent channels for non-overlapping time intervals, and each channel has its own modulation applied thereto.

A brief description of the complete system of the invention utilizing transmitting `and receiving equipment will now be given.

In the transmitting portion of the equipment, the modulating signals from a number of separate audio inputs are combined into a complex l5 risers from one step voltage wave occur at a difpulse type wave sometimes called a pulse train ferent time than risers in the other step voltage or video frequency Wave. That is, each channel Waves and each riser causes one channel in the sequentially produces a pulse a characteristic of system to become operative and produce a moduwhich is modulated in accordance with the modlated pulse. ulating signals applied to the channel. One 2o In systems involving a relatively small numframe or cycle of operations includes one pulse ber ofchannels (up to a single step voltage from each message channel and a synchronizing wave is sufficient, in which case the number of pulse which has a characteristic distinct from the risers in the step voltage wave is usually made pulses of the message channels, such as longer equal to the number of channel units employed. duration or higher amplitude, and which allows However. when a larger number of channels is the receiving equipment to separate it from the involved, a single step voltage wave becomes imchannel pulses. The complex pulse type wave is practical from the standpoint of stability of opcoupled to a radio frequency transmitter which, eration and design, This invention overcomes in turn, feeds a suitable wave radiating structhese diiliculties by producing a number of interture. laced step voltage waves so interlaced in time The receiving portion of the equipment inf that each riser in the diilerent step voltage waves cludes a radio frequency receiver which is tuned occurs at a time which is different from and beto the frequency of the remotely located radio tween the times of occurrence of the risers from frequency transmitter and which re-establishes any other step Voltage wave. Each step voltage on its video output terminals the complex pulse wave is coupled to a group of channel units, the type Wave which is then passed on to the receivnumber of channel units in each group being ing multiplex equipment. The receiving multiequal to the number of risers in the applied step plex equipment selects the synchronizing pulse voltage wave, and the number of groups being and from it generates a series of gate pulses equal to the number of interlaced step voltage which make the receiving channel units operate wavesgenerated. sequentially and at a time when the modulated A more detailed description of the invention pulse assigned to the channel is present on its .follows in conjunction with drawings wherein: video input terminals. In other words, the com- Fig. l illustrates, diagrammatically, the transpleX pulse type Wave is coupled to all receiving mitting portion of the time division multiplex channels which have their Vdeo frequently Ilsystem of the invention utilizing plus (-i) and puts connected in an electrically parallel fashminus amplitude modulated channel pulses ion, and the channels are made operative one at which are applied to a frequency modulated atime, when the pulse assigned to a particular radio frequency transmitter. V channel is present on its video input terminals, Fig. 2 illustrates, diagrammatically, the receivby means of gate pulses which are controlled by ing portion of the system. M

the received synchronizing pulse.

.The system herein described utilizes amplitude modulated channel pulses accompanied by a longer duration synchronizing pulse. The synchronizing pulse has an amplitude equal to the peak amplitude of the modulated channel pulses. The radio frequency equipment is of the frequency modulation type. Other types of pulse modulation such as pulse width, pulse numbers or pulse position may be employed, utilizing the timing equipment of this invention, in which case the radio frequency equipment would be of the type most suitable for the type of pulse modulation employed.

More specically, the System of the invention enables the allotment of time intervals to diierent channels sequentially and for non-overlapping time intervals by the production of multiple step voltage waves in such a manner that the Figs. 3a and 3b taken together, illustrate, yschematically, the cricuit details of the transmitting multiplex common equipment for the plus and minus pulse amplitude time division system.

Fig. 4 illustrates, schematically, the circuit details of a transmitting channel unit which may be used with the common equipment of Figs. 3a and 3b.

Fig. 5 is a series of waveforms, curves 5a to 5q, graphically illustrating the voltage waveforms at different points of the circuits of Figs. 3a, 3b and 4.

Figs. 6a and 6b taken together, illustrate, schematically, the circuit details of the receiving multiplex common equipment for the plus and minus pulse amplitude modulated time division multiplex system.

Fig. 7 illustrates, schematically, the circuit details of a receiving multiplex channel unit which may be used with the common 'equipment `of Figs. 6a and 6b.

Fig. 8 is a series of waveforms, curvesa to 8x, graphically illustrating the voltage waveforms at different points of the circuit of Fig's. 6a, 6b and '7.

Referring to Fig. 1 in more detail, this figure shows the transmitting apparatus for a plurality of channels, each of Which is supplied with its own modulating signal.

General description of transmitter operation AA crystal controlled pulse oscillator A producing positive pulses as indicated by waveform 80| is employed to drive a master step voltage wave generator B over leads |00. The step voltage wave output from B as indicated by waveform 802 is coupled to counter D and substep Voltage wave generators E, F, and G via lead |I.

The counter D counts a predetermined number of stop risers in the step voltage wave output from B and produces positive pulses on its two output leads |02 and |03, as shown in waveforms 803 and 804, respectively, after the predetermined number of top risers have been counted.

Each substep wave generator has two output terminals, on one terminal of which there is produced a step voltage wave and on the other a short positive .pulse (short compared to the period of the pulse oscillator A) occurring once for a particular riser in the step voltage wave output from B. That is, each substep wave generator E, F and G contains a selector tube circuit which is biased to become conducting on a particular riser in the step voltage wave output from the master step wave generator vB and produces a pulse which causes a riser in the output of the substep wave generator to occur, and after a `short time delay a positive pulse is impressed on the other output terminal. The time delay between the time of occurrence of the riser in the step voltage wave output and the occurrence time of the pulse is made short compared to the period of the pulse oscillator A for reasons which will be explained later. The selector tube circuits in the substep wave generators are biased to become operative on different risers in the step voltage wave output from the master step wave generator B. Stated in other words, 'the step wave outputs from the substep wavegenerators are interlaced in a manner clearly shown inthe curves d to 5f of Fig. 5 to be described hereinafter.

The operation of the substep wave generators is explained as follows: On each .number riser in the step voltage wave output from'the master generator B as indicated by waveform 802, the selector tube circuit in substep wave generator E produces a pulse which in turn produces a riser in its ovvn step voltage wave output von lead |01 as indicated by waveform 808. After a short time, a positive pulse as indicated by waveform 801 is produced on output lead |04. On each number 2 riser in the step voltage wave output of B, the selector tube circuit in substep wave generator F produces a pulse which in turn produces a riser in its own step voltage Wave ouput, as indicated by wave form 8I0, on lead |08. After a short time a positive pulse, as indicated by Waveform 809, is produced on lead |05. And on each number 3 riser in the step voltage wave output from the master generator B, the selector tube circuit in substep voltage wave generator G produces a pulse which in turn produces a riser in its own step voltage wave on lead |09, as indicated by wave form 8| I. After a short time, a positive pulse, as indicated by waveform 8I2, is produced on lead. |06.

The counter D also contains a selector which is biased to become conducting on the last or number 3 riser from the step wave voltage output from master generator B and which after a predetermined number of last -or number 3 risers from the step wave outputs from B causes a positive pulse as indicated by waveform 804 to be developed on lead |03. This pulse discharges all substep wave generators simultaneously. At the time of occurrence of the pulse 804 on lead |03, another pulse as indicated by waveform 803, is produced on lead |02 by counter D. This pulse causes the synchronizing pulse generator C to produce a synchronizing pulse on lead I I0 which is of longer duration than the final channel pulses.

The pulse outputs 'from substep Wave generators E, F, and G lare coupled to 'the plus and minus PAM converter S by means of leads |04, |05 and |06, respectively, and the step voltage wave outputs from these substep wave generators are coupled to channel banks H, J, and K via leads |101, |08, and |09, respectively. Each channel bank includes a vplurality of channel units.

Each transmitting channel unit consists of a position selector, a gate generator, and a pulse modulator. The channel position selectors are differently biased to become operative on different risers of the applied step voltage wave from the associated substep wave generator, and yeach drives a gate ygenerator which produces a gate pulse which allows the pulse modulator to produce a negative, amplitude modulate pulse on an output lead common to all channel modulator tubes in the channel bank. The output from each bank -of channel units consists of a series of negative, amplitude modulated, sequentially occurring pulses which are coupled to the plus and minus PAM converter unit S. Thus the output pulses from channel bank H (which comprises four Vchannel units I, 4, and I0) are coupled to PAM converter S by way of lead I I2; the output pulses from channel 4bank J (which comprises four channel units 2, 5, 8 and II) are coupled to converter S by way of lead III; and the output pulses vfrom channel bank K (which comprises three channel units 3, 6, and 9) are coupled to converter S by way of lead I I0.

The plus and minurn .PAM converter unit 'S converts the relatively longer duration and negative amplitude modulated channel pulses on leads H0, III and II2 to positive and negative shorter duration vamplitude modulated pulses with a duration equal to the duration of the pulses applied on leads |04, |05 and |06. VThe pulses in leads II'0, `I II and II2 are diagrammatically illustrated by waveforms SI5, 814 and 8I3, respectively.-

The double arrows on these waveforms indicate amplitude variations. The output of 'the plus anarco.

and minus PAiVI converter' S consists of a series of pulses, the amplitude and polarity of which are a function of the instantaneous'amplitude and polarity of the modulating signal applied to the channel at the occurrence time of the pulse. If the modulating signal has zero amplitude at the time the channel produces a pulse, then there will be no output pulse from the PAM converter' S for that channel. In other words, the pulse produced by converter S for a channel unit having no modulation applied thereto will have zero amplitude.

The output from the PAM converter S as indicated by Waveform SI2 is coupled to one input of a video amplier L via lead I I3, and the synchronizing pulse as indicated by waveform 805 is coupled to another input of video amplifier L via lead IM. The output of the amplifier L consists of a train of channel pulses followed by a longer duration synchronizing pulse as shown in waveform 800 and is coupled to the frequency modulation transmitter T via lead H5. Each pulse causes the frequency of the transmitter T to be deviated by an amount proportional to the amplitude of the applied pulses. Stated otherwise, the converter S converts the negative going amplitude modulated pulses received by it over leads III?, III and II 2 to plus and minus amplitude modulated-pulses and feeds these last pulses to the common video amplifier L. The dashed line pulses in Waveform 800 and 8 I 2 indicate the maximum positive and negative amplitudes which the channel pulses may attain on the extremes of modulation. The radio frequency output from the transmitter T is coupled to a suitable radiating element V via transmission line TL.

General description of receiver operation Referring to Fig. 2, which shows diagrammatically the receiving multiplex equipment, the radio frequency `Waves are picked up on antenna V-I and are coupled to a radio frequency superheterodyne receiver 20| Via transmission line TL-I. The radio frequency receiver dernodulates the frequency modulated radio frequency signals and re-establishes on its video output terminals the pulse train as shown in Waveform lill. The output of receiver 23I is coupled to a video amplifier 202 via lead 301. The output from the video amplifier as indicated by Waveform '|04 is coupled via lead 30'@ to all receiving channel `units in the various channel banks which have one of their inputs connected in an electrically parallel manner. The'output from the video amplifier 202 is also coupled to a synchronizing pulse selector 203 via lead 3i2. The synchronizing pulse selector 203 selects or separates the synchronizing pulse from the channel pulses by virtue of the longer duration of the synchronizing pulse and produces a pulse which occurs once for each received longer duration synchronizing pulse as shown by Waveform 505. The output from the synchronizing pulse selector 203 is coupled to a synchronizing pulse amplifier 204 via lead 303. The synchronizing pulse amplifier has two outputs, one of which is connected to a phasing circuit 205 via lead 336 and the other to a master step Wave generator 2II and substep Wave generators 308, 20S and 2i@ via lead 308. The synchronizing pulse output appearing in lead 308 is shown by Waveform 102.

The phasing circuit 235 produces a pulse whose occurrence time is made to be manually adjustable. That is, the occurrence time of the pulse produced by the phasing circuit 205lags the sea lected synchronizing pulse by a time interval which is manualy adjustable to compensate for time delays in the circuits of the receiving equipment. Of course, this adjustment could be made automatic With some complication of the receiving equipment. f

The positive pulses fromlthe phasing circuit 205 as indicated by waveform I0@ are coupled to a tuned circuit 206 which produces a slightly damped sine wave of a frequency equal to that of the crystal in the transmitting multiplex equipment. This damped Wave as indicated by Waveform '01 is coupled to and locks in a pulse oscillator 20T, resulting in the production of pulses with a repetition rate equal to the repetition rate of the pulses produced by the pulse oscillator A KVVin the transmitting multiplex equipment (Fig. l).

, The positive pulses from the pulse oscillator 201 as indicated by Waveform 108 are coupled via lead 3I3 to a master step Wave generator 2II Which produces on its output terminals a step voltage wave as indicated by waveform il I.

The step Wave output from the master step Wave generator is coupled via lead 309 to three substep Wave generators 208, 200, and 2&0, which produce step Waves on different risers of the step wave output from 21| in a manner similar to that described for the substep Wave generators E, li' and G in the transmitting equipment.

The step voltage Wave outputs from the substep Wave generators 2I0, 209, and 208 are coupled to three banks of channel units H-I, J-I, and K--I via leads 3I2, 3| I, and 3I0 respectively. Channel banks H-I, J-I and K-I correspond to channel banks H, J, and K at the transmitter, Fig. l. The respective step voltage Wave outputs from substep wave generators 208, 209 and 2I0 are represented by waveforms 703, 109 and H0.

Each receiving channel unit in each bank includes a channel position selectora gate generator, a 10W pass iilter, and an audio amplifier. The channel position selectors are differently biased, in a manner similar to that described for the transmitting channel selectors, to become operative on different risers of the applied step waves. When a channel position selector becomes operative', it causes a gate generator to produce a pulse which renders an amplifier tube operative, thus allowing it to pass the desired pulse. The length of the gate, and hence the length of time during which the amplifier tube is made operativeis made equal to the duration of the video channel pulses. The channel pulse thus selected is coupled to a low pass filter which attenuates the pulse frequencies and allows the modulating frequencies to pass. The output from the low pass lter is coupled to an audio amplifier which amplifes the audio frequency signals and couples them to the output terminals. The Waveform of the modulating frequencies in the outputs of the channel units is represented by Waveform Details of transmitter and resistors 340,' 34| and 344. The pulse oscilj lator sectionincludes a normally' non-conducting tube 347, a pulse transformer 345, condenser 346', and resistors 34| and 342. y

The operation of these circuits is as follows: Crystal 33? together with condensers 338 and 339 form the oscillatory circuit with condensers 338 and 339 acting as a voltage divider to provide the proper relationship between the anode and grid voltages. On each positive peak of the sine wave across condenser 339, tube 343 draws current, resulting in a positive voltage pulse being developed across resistor 34| and a pulse of current applied to the oscillator circuit sufficient to maintain oscillations. The positive pulses thus developed across resistor 34| are usedv to lock in the pulse oscillator.

The pulse type transformer 345 in the anode circuit of normally non-conducting tubeA 34"!" is so poled that as the anode current in vacuum tube 341 increases, the voltage applied to the grid increases, resulting in a further increase in anode current. This action continues until the anode current in tube 34l' reaches a maximum, at which time there is no further increase in grid voltage. Hence the voltage on the grid starts to decrease, resulting in a reversal of the above events, until tube 341 is completely cut-olf. Tube 34T remains cut-01T due to a Voltage developed across resistor 342 due to condenser 345 discharging. Condenser 346 is charged by means of grid current during the pulse time. The values of condenser 345 and resistor 342 are so chosen that tube 34'! is nearly ready to conduct at the occurrence time of the positive pulse across resistor 34| due tothe action oi the crystal oscillator. This positive pulse causes tube 341 to start to conduct and the above described action takes place. In other words, the frequency of the pulse oscillator is set to be slightly lower than the frequency of the crystal and therefore each pulse across 34E developed by the crystal circuit trips the pulse oscillator and causes it to produce a pulse. It will thus be noted that the pulse oscillator section of A can be considered as a transformer feedback blocking oscillator controlled by voltage pulses developed across resistor 34|.

The operation of the master step wave generator B, which includes normally'non-conducting vacuum tubes 35| and 354, rio1"mally'con ducting tube 355, pulse type transformer 353, resistors 349, 352-, 35E and 358 and condensers 348, 353 and 351 is as follows: The winding 39| ofV pulse transformer 345 is so poled that each time the pulse oscillator produces a pulse, a positive pulse is applied between the grid and cathode of tube 35| causing it to conduct and draw grid current, charging condenser 343. Afterthe pulse from the pulse oscillator ceases, tube 35| cuts-oil" and remains cut-off dueto grid leak bias developed across resistor 349. Eachtime tube 35| conducts, an incremental charge'is stored in condenser 350, resulting in a step of voltage being developed there-across. The magnitude of the incremental charge and hence the magnitude of the incremental changes in voltage across condenser 359 is a function of theV duration of the pulse from pulse oscillator'A, the impedance offered by tube 35| and the values of resistor 352 and condenser 35|). Since the pulse length and the impedance offered by tube 35| remain constant, the values of condenser 350 and resistor' 352 may be chosen such that a desired amplitude of incremental voltage change across 350`foreach pulse from pulse oscillator A, may be had. Since tube- 354 is non-conducting and tbe 35| is a" cathode follower, the voltage developed across condenser 350 remains until the next following pulseV from A, that is, each change in voltage is added to the voltage developed by previous pulses fromv A, resulting in a step voltage wave as shown in Fig. 5b (Fig. 5a represents the pulses from pulse oscillator A).

When the combined amplitude of the steps of voltage across condenser 350 exceeds the bias on normallyy non-conducting tube 354, as indicated by thek horizontal dash-dot line X, through curve 5b of Fig. 5, tube 354 starts to conduct. Transformer 353 is so poled that when vacuum tube 354 starts to conduct a positive voltage is applied to the' grid of tube 354 which further increases the current. This action continues until grid current flows through the grid Winding of transformer 353 and discharges condenser 350 at which time tube- 354 again cuts olf. Tube 354 is biased below' cut-oil by means of resistor 356 and condenser 351 in its cathode circuit. That is, each time' tube 354 carries current, a charge is stored in condenser 351, and during the time interval during which 354 is cut-off, this charge starts to leak olf through resistor 356, resulting in a positive voltage being developed there-across. Tube 355 is a conventional cathode output amplier which offers a high input impedance to condenser 35) and a low output impedance for lead |El|.

Curve 5b of Fig. 5 shows the step waves as they appear on the-output lead |il| of the master step wave generator B. It will be noted that there are three risers as shown. This will supply three substep wave generators. If more substep voltage waves are desired, then the number of risers in waveform 5b should be increased accordingly. At the present state of the art, a practical limit of countingis 15, hence this circuit could supply a step voltage wave suitable for use with l5 substep wave generators, each substep wave generator producing step voltage waves of 15 risers each. This would allow spaced timing facilities for 224 channels.

The counter D, which is used to discharge the three substep wave generators E, F and G, includes three sections: (l) a selector section which contains a pulse transformer 302 and a normally non-conductivefvacuum tube 304 which is biased to become operative on the last or top riser 0f the step voltage wave from the master step wave generator B, (2) a one shot pulse generator consisting of a normally non-conducting vacuum tube 3|2 and pulse transformer 3|4, which negatively chargescondenser 3|0 after a predetermined number of top risers in the master step voltage wave have made the selector operative and (3) a discharge section consisting of a normally non-conducting vacuum tube 337 which removes incremental charges from condenser 3|?) each time the selector section is made operative.

The operation of this unit is as follows: The step voltage wave as shown in curve 5b is coupled to the grid of tube 364, via lead |0|. Tube 304 is biased, by means of cathode bias, to become operative on the last riser as shown by the horizontal dash-dot line (x) in curve 5b. It should'be noted that the last or top riser of this step voltage wave is a pulse due to the fact that the discharge of the step voltage wave occurs im mediately following this riser. When the step wave riser exceeds the bias potential on tube 334 it conducts and a pulse of current flows through the anode winding of transformer 392, tube 3mi-and cathode condenser 305. When tube 304ficeases-conducting, the current stored in cathy ducts.

ode condenser 305 starts to leak off through resistor 306, thus developing the desired bias voltage. vCondenser 305 is `made suiciently large in order that the change in voltage across resistor 306, during the time when 304 is non-conducting, is small compared to the D. C. bias voltage developed. Transformer 302 is so poled that a positive pulse is applied between the grid and cathode of discharge tube 301- each time 304 con- This positive pulse causes tube 301 to conduct and remove an incremental charge from condenser 3I0, the amount of charge removedV being a function of the duration of the Vlast riser in the master step voltage wave, the impedance of tube 301 when it is conducting and the values of resistor 308 and condenser 3I0. By properly choosing resistor 308 and condenser 3H), the desired value of incremental charge may be removed. Each time tube 301 conducts, grid current flows, charging condenser 303, and this charge then leaks off through resistor S and develops suicient bias to maintain tube 301 cutoiT during the inoperative periods of the selector tube 304. After a predetermined number of top risers, the voltage developed across condenser 3| 0 is reduced to` a value which permits normally non-conducting tube 3I2 to start to conduct. Transformer 3| 4 is so poled that when current starts to ow in tube 3I2 a positive voltage is Vapplied to its grid which further increases the current. This action continues until the anode current no longer increases at which time the positive .voltage applied to the grid of tube 3I2 starts to decrease, resulting in a decrease in anode current which in turn further decreases the grid voltage. This action continues until tube 3I2 is again cut-off. Tube SI2 remains cut-01T due to the negative voltage developed across condenser 3 I 0 as a result of grid current which followed and negatively charged condenser 3 I 0 during the con-Y ducting time of tube 312.

On the next top riser vin the master step voltage wave, tube 304 conducts which in turn makes tube 301 conduct and remove another incremental charge from condenser 3I0. This action continues until the voltage across condenser 3I-0 is again reduced to the cut-off potential of 3I2. The voltage waveform as it appears across condenser 3I0, is shown in curve 5c of Fig. 5. It should be noted that the step wave developed across condenser 3I0 never reaches zero amplitude since, when it exceeds the cut-off potential of tube 3i2, as indicated by the horizontal dashdot line Eco, it is again reduced to a high negative value. By properly choosing the values of condenser 3I0. and resistor 308, the desired number of top risers may .be counted. Each time tube iz'conducts., a positive pulse is developed across its cathode resistor 3l I. This pulse is coupled to thesynchr'onizing pulse generator C, via lead |02. A positive pulse is also developed across the output winding of transformer 3I4 each time tube SI2 conducts, and is coupled to the discharge tubes in substep wave generators E, F land G via lead I03.

Since the three substep wave generators E, F andi@u are identical with the exception or" the values of the selector tube cathode resistors, the operation of only substep wave generator E will `rie-.described in detail.v Similar components in the other substep wave generators F and G have been given the same reference numerals with prime and double prime notations. Each substep wave generator consists of ve sections: (l) a position selector which produces a pulse to lock in a pulse oscillator in the? proper time relation With the master step wave;A (2) a pulse oscillator which produces" two simultaneously occurring pulses, one to drivel a step charge tube and the other to drive the i- PAM converter unit; (3) a step charge section which stores incremental charges in a step charge condenser; (4) a step Wave discharge section which removes the charge stored in the step charge condenser; and (5) a cathode output amplifier which couples the step voltage wave to the various channel units in the channel bank assignedto that particular substep wave generator. y

The operation of substep wave generator E is as follows: The masterystep voltage wave from Agenerator B is coupled to grid limiting resistor 3I1 via lead lill. The other end of resistor 3I1 isconnected to the grid of normally non-conducting selector vacuum tube 322, which is biased to become conducting at approximately 1/3- of the amplitude of the iirst riser of the master step voltage wave, as indicated by the horizontal dashed line y passing through the number one riser of the waveform ,5b in Fig. 5. When the amplitudeof the number one riser reaches a value as indicated by the horizontal dashed line Z, the grid to cathode potential of tube 322 reaches zero. It remainsv at this .value for the remainder of the step Voltage wave cycle due to grid current flowing in resistor 311, and creating a voltage drop which Iapproximately equals the increase in the step voltage Wave above line Z. That is, as the kstep wave on lead |0I increases above a value indicated by line Z, the grid current in tube 322 increases and produces a voltage drop across resistor 311,A approximately equal to the voltage increase on lead I0! abovethe value Z. Thus, cur- .'rent in tube 322 starts 'to :How and reaches a maximum on the rst riser cf the master step voltage Wave. The current in tube 322 remains at this lmaximum value until the vdischarge occurs, at

which -time tube 322 again cuts off. Cathode bias is supplied to tube 322 by means of resistor 318 and condenser 3I9 as previously described for selector tube 304 in counter D. When tube 322 starts toconduct, a charge is stored in condenser 324, resulting in a pulse'of current in transformer 328 which is so poled as to apply a positive voltage pulse to thefgrid of normally non-conducting vacuurn tube 326, of a value suicient to cause current flow therein. When current ilow starts in tube 32 6, the grid voltage is further increased due to the connections of ltransformer 328 and hence, the anode current further increases. This action continues until the anode current of tube 325 reaches a maximum at which time the voltage applied to its grid starts to decrease which in turn decreases the anode current. This action continues until tube 326 is again cut-off. When tube 326 conducts, gridv vcurrent flows, charging condenser 325, and during the time interval between successive risers in the master step voltage wave signal. This charge leaks 01T through grid leak resistor 320', developing a negative voltage thereacross, suicient to maintain tube 325 non-conducting. After the initial flow of current in tube 322, its anode potential drops to a low value due .to a voltage drop developed across resistor 323.

The Value of resistor 323 is chosen to provide the proper operatingconditions for tube 322; that 1s. to reduce the voltage to a value where the anode power is within the limits required by the manufacturer for the type of tube used. The value of condenser 324 is made such that it completely discharges through resistor 323 during the time interval when tube 322 is cut-oli.. Each :time -tube 326 conducts., a voltage `pulse Adevelopedacross the cathode winding -of .diierentiating transe former 321. Resistor .32| -is 'a damping resistor to critically damp 321. The waveform of the pulse appearing across the secondary winding of differentiating ytransformerZ 1, and Aappearing -cn lead 64 is shown in Icurve 5g of Fig. 5.

Winding 33| of transformer328 is so poled that a positive voltage isapplied between the grid and cathode .of vacuum tube `3.32, .causing it to conduct and store `an Aincremental charge in condenser 333. Tube 332 is biased below cutoi by means of grid leak bias developed across resistor .329, during the time .interval .between successive pulses from transformer 328as described for tube 36'. in counter D. Resistor 33| and .condenser 333 are chosen to give the .desired .incremental voltage step foreach pulse from 32.8. .The voltage across condenser 3,33 increases in steps as shown in curve .5d in Fig. 5, until the counter D yproduces a pulse which causes normally non-.conducting tube 333 to become .conductingand discharge .condenser 333. Tube 334 is normally biased below cut-off due .to grid leak bias developed :across resistor 3 i6 (shown .in the counter D section) which results from the discharging .of condenser" 3E5 therethrough. 4.Condenser 3175 v-is charged .by grid current from tubes 334, 334' `and 334 .each time the counter D produces a pulse.

Tube 335 .is a normally conducting cathode output vacuum tube amplifier which couples the step wave thus developed t the channel bank H. The step waves developed'by substep wave generators E, F and C- are interleavedas shown .in curves 5d, 5e and 5f of "Fig, 5, respectively. 'It should be noted that the risers for substep wave Agenerator E occur at the time of each number riser in the master step voltage wave.; the risers of substep wave generator Foccur at the time of each number 2 riser of the master step voltage wave.; and the risers of substep wave generator G occur at the time of the number '.3 riser of the master step wave, and also note that the discharge of all substep voltage waves occur simultaneously and at the time when the counter D produces a pulse. Counter D counts four complete step waves produced by master step wave generator B.

The pulse developed across the secondary wind ings of differentiating transformers 321, 321 and 321 in the different substep wave generators E, F and G are as shown in waveforms 6g, 5h, and 5i, respectively. It is to be noted that the positive peaks of these pulses are delayed by an amount determined by the oscillatory frequency of the transformer after the occurrence time of 5g of Fig. 5 are coupled `to the grid of normally.

non-conducting vacuum tube 362 via lead |03. Tube 362 is normally biased below cut-'off by means of bias voltage developed across cathode resistor 359. Condenser 36|) vmaintains the bias voltage across resistor 359 during the noncon-` ducting period .of tube 363. The horizontal dashed line through the waveform of Fig. 5g indicates the potential above which tube 362 conducts. When tube 362 conducts, a negative pulse is developed across .resistor 36.3. This negative pulse is coupled to .the anode .of .normally conducting diode l36.6 lvia .coupling .condenser .36.5 and .renders diode -366 non-.conducting for the duration of the pulse. Current normally flowing through diode 366 develops .a voltage across resistor `36|, which is .also in the cathode circuits of vacuum .tubes 361 and 368, and this voltage is sufficient to render tubes 361 and 368 nonconducting. The value of resistor .36| is so chosen that when .diode .366 .cuts off, .tubes .36.1 .and 368 become conducting and operate as class A ampliers. .That is, .during the conducting period of tubes 361 and 368, they operate on .the linear portions of their respective grid .voltage-anode current characteristic curves. Resistor 364 has the proper value .to provide .the desired bias voltage across resistor 36| when diode 366 is conducting.

The negative amplitude modulated channel pulses from Ylead ||2 are coupled through coupling condenser` 31| to .the ,grid of tube 368. A grid leak 369 is connected between the grid of tube 368 and a tap on potentiometer 31D which forms part of a bleeder network between -l-Ebb and ground. The tap on potentiometer 310 is set at a value such that the grid-to-ground potential .on tube 368 is zero when an unmodulated channel pulse is present on lead |2. Hence, the grid-toground potential varies positive and negative with respect to ground or .zero potential when the channel pulses are modulated. Since .the grid of tube 36.1 is directly connected to the ground, the

`potential on the grid of .tube 368 varies plus and minus about the potential on the grid of tube 361, resulting in the peak amplitude of .current in tube 368 being greater or vless'than the current in tube -361 by an amount proportional to the unbalance in the grid voltage. The current in tube 361 flows vfrom +Ebb lthrough resistor 313, and the current in tube 368 iiows'through resistor 314. It should be noted that the `anodes of tubes 361 and 361 are also connected to resistor 313 and that the anodes of tubes 368', 368" and 315 are connected to resistor 314 in electrically parallel relationship with 368.

The current in tube 361 produces negative pulses across resistor 313 which are coupled to normally conducting vacuum tube 315, via cou- 'pling condenser 316 and renders it non-conducting for the duration ofthe pulses. The bias on tube 315 is set by means of cathode resistor 318 to a value such that the amplitude of current in tube 315, when it is conducting, is equal to the amplitude of current in tube 368 when it conducts with unmodulated channel pulses present on its grid. Condenser 319 maintains this bias for the time interval during which tube 315 is cut-01T. Since tubes 361 and 368 become operative simultaneously and the pulse developed across resistor 313 due to current in tube 361 drives tube 315 below cut-off, it will be seen that the decrease in current in tube .315 is equal to the increase in current in tube 3.68 when Athe pulses applied to the grid of tube 368 are unmodulated. Therefore, since the anodes of tubes 368 and 315 are connected in parallelv to resistor 314, the increase in current therein due to tube 368 vbecoming operative is exactly balanced by the decrease in current due to tube 315 becoming inoperative, resulting in no 'voltage change across resistor 314 when unmodulated channel pulses are present on the grid of tube 368.

If, however, the grid of tube 368 were more negative than the grid of tube 361 when they became operative, due to the channel pulse being cfa higher negative value, then the increase in current in tube368 would not be as great as the increase in current in tube 361. Therefore, the increase in current in tube 361 would not be as great as the decrease in current in tube 315, resulting in a positive pulse being developed across lresistor 314. By similar reasoning, it may be shown that, when the negative channel pulse has an amplitude less than the unmodulated value, a negative pulse will be developed across resistor 315|.

Since the anodes of all tubes 361, 361' and 36'!" are connected to resistor 313, and the anodes of tubes 368, 358 and 358 and 315 are connected to resistor 31d, only one tube 315 is necessary to balance the three sections 71,7' and k.

The positive and negative pulses thus developed across resistor 314 are coupled via condenser 38| fand lead H3 to the grid of normally conducting cathode output vacuum tube amplifier 334 which has a cathode resistor 382 in common. with another cathode output tube 385. Tube 384 couples the pulses to output lead H5 which is connected to the R. F. transmitter.

The positive pulses developed across cathode resistor 3| Ain counter D, after counter D counts a predetermined number of top risers in the master step voltage wave from the master step wave generator B, are coupled to the grid of a normally conducting synchronizing pulse gener- 'at'or vacuum tube 381, via lead |02 and condenser T339." The operation of the synchronizing pulse 'g'enerator is as follows: Resistor 390 located be- 4'tween the gridof tube 381 and Ebb is of very 'fhighvalue (of the order of l megohm) and maintains the grid-to-cathode potential of tube 381 at approximately zero, and when the positive pulse cn lead HB2 is present, condenser 399 is charged L'through the low impedance of the grid-to-cathode circuit., When the pulse on lead |02 ceases, fthe charge stored in condenser 389 starts to decrease through resistor 393, developing thereacross a negative Voltage suiiicient to render tube 331 non-conducting. The duration of the noncenducting period of tube 381 is determined by the timeV required for the charge stored in condenser 389 to leak ofi` through resistor 390. The values of condenser 389 and resistor 399 are vchosen to make this time longer than and preferably equal approximately to three times the "duration of the channel pulse. The value of resistor 39@ is chosen to be of a value which al- Alows'the lpower dissipated in the grid of tube 331 ,to bev within the specifications presented by manufacturer. 'I A positive pulse, as shown in waveform 5p "of Fig. 5, is developed across resistor 388 as a A result of tube 381 becoming non-conducting. This positive pulse is coupled to the grid of normally "non-conducting vacuum tube 385 (video amplier), via coupling condenser 385, rendering tube 335 conducting and thus developing a positive -pulse on lead H5. Tube 385 is rendered non- *conducting during the time interval between successive synchronizing pulses due to the discharge through resistor 383 of a charge stored in condenser 336. The charge stored on condenser 335 is caused by grid current from tube 395. When vv'condenser 386 discharges through resistor 383 there is developed a negative bias across refsistor 333 which is sufcient to supply the cutoi potential required. f

The synchronizing pulse, curve 5p. occurs at ;a time immediately following the step wave dis- .'charge and when `no channel.. pulses are'. being 14 produced by the channel units.H The combined channel and synchronizing pulses, as they appear on lead l5, is as shown in curve 5g of Fig. 5.

Time sequence operation of transmitter A complete description of the operation of the circuit with respect to time will be given as follows: In order to include the operation of the step discharge and synchronizing pulse generator, and to simplify the description, the operation beginning at time R which is indicated by the vertical dash-dot line R through the curves of 5a to 5q of Fig. 5 will be given. Assume the chanlnel pulse 9 in curve 5o (from channel 9 and appearing on lead H0 is at its maximum negative value as indicated by the lower dotted line. Also assume the negative pulse for channel I0 (from channel Hl and appearing on lead H2) is unmodulated as indicated by the solid line pulse II) in curve 5m, and further assume that the pulse for channel (from channel and appearing on lead I is at its minimum negative value as indicated by the upper dotted line for pulse in wave form 5u.

A pulse from the crystal controlled pulse oscillator A occurs at time R (see pulse 9 in curve 5a) and causes a riser in the master step voltage wave to occur as shown in curve 5b. This riser causes the master step voltage wave to discharge and start a new counting cycle.

The riser in the master step voltage wave caused by pulse 9 in curve 5a causes a riser to occur across the condenser in the counter D as shown in curve 5c and also a riser to occur in the step voltage wave output of the sub step voltage wave generator G as shown at 9 in curve 5f. The riser 9 in curve 5f is coupled to channel bank K, which causes channel 9 to produce on lead IH) a negative pulse, as shown at 9 in curve 5o. This negative pulse is coupled to the grid of tube 368 which is biased below cut-01T; hence. nothing happens in the anode circuit of 368" at this time. A negative pulse as shown in curve 5i is also developed across the secondary winding of differentiating transformer 321" in the step wave generator G, at the occurrence time of the riser. A short time later, the pulse across the secondary winding of 321 reverses and rises above the cut-off bias on tube 362" in the iPAM converter S causing it to conduct and, in turn, produce a negative pulse on lead |536 sufcient to cause diode 356" to cease conducting which, in turn, allows tubes 361" and 368 to become operative by reducing their common cathode bias voltage, as shown at .9 in curve 5l. Tube 315 in the converter S becomes non-conductive due to the negative pulse developed across resistor 313 (caused by current in tube 2361") and since the pulse is assumed to be at its most negative value, the increase in current in tube 358" is smaller than the decrease in current in tube 315. Hence, a positive pulse is developed across resistor 314, which in turn is coupled to lead H5 through the coupling tube 384. This pulse is shown by the positive portion of the dotted pulse 9 inwaveform 5g. After a period of time equal to theduration of the positive pulse 9 above the dashed line `in curve 5i, tubes 361 and 3581' again cut-01T and tube 315 again becomes conductive. A short time later, the negative channel pulse 9, curve 50, ceases and again, since 368 is cut-off, nothing happens.

On the next following pulse from the pulse oscillator, as shown at l0, curve 5a, a number one riser occurs in thefmaster step voltage wave 115 .of curve 5b. This :riser causes selector tube 322 insubstep voltage wave generator E to produce a pulse, which in turn produces a riser Ain the step voltage wave output of E, as shown at Iii in curve 5d, which in turn causes one channel in bank H to produce a pulse which is coupled to the grid of tube 368 in .section H of the i PAM converter` S. Since at this time 353 is `cut-oi, nothing new happens. At the occurrence time of ,riser I0, `a negative pulse is developed lacross the secondary winding of diierentiating transH former 321 in the cathode of tube 326 in substep Wave generator El. After a short Vperiod of time, determined by the natural frequency of .321, a

positive pulseas indicated .at IG by Waveform 5g exceeds the cut-off potential of tube 352 in section h of the i PAM converter rendering it conducting and thus producing a pulse which causes normally conducting diode 36.5 to become non-conducting, thus developing a negative pulse as shown in waveform 5j across resistor 36I which in turn allows tubes 35'! and 368 to become conductive. rlube 361 develops across resistor 313 a negative pulse which again makes tube 315 non-conductive, andsince it is assumed that the l negative pulse produced by channel I5 is unmodulated, the increase in current in tube 368 is exactly 'balanced by the decrease in current in vtube 315, resulting in no change in voltage across resistor 314. Hence, no pulse is present on lead II5 at this time, as shown in the solid line Yat I0 in waveform 5g. When the pulse applied to diode 355 ceases, tubes 361 and 358 again outo and tube 315 conducts. A short time later the negative channel pulse applied to the grid of tube 368 ceases, but since tube 358 is cut-ofi" at this time, nothing happens.

On the next following pulse from the crystalcontrolled pulse oscillator A, as shown at H, waveform 5a, the #2 riser in the master step voltage wave occurs, as shown in waveform 5b. `This causes selector tube 322 in substep Wave generator F to become conductive, which results in a riser as shown at II in curve 5e. This riser causes channel II in channel bank J to produce a negative pulse as shown at I I in curve 511, which in turn is coupled to the grid of tube 368 in section J of the i- PAM converter unit S. Since at this time tube 368' is cut-off, nothing happens.

At the time of occurrence of riser number II, a

negative pulse as shown at II in waveform 5h is developed across the secondary winding of differentiating transformer 321 in the cathode of tube 326 in substep wave generator F and is coupled to the grid of tube 362 in section J of the i PAM converter S. After a short period of time determined by the natural frequency of 321 the pulse across the secondary winding of 3.21' reverses and exceeds the cut-off potential of tube 362' asindicated by the horizontal dashed line in waveform 5h. This causes tube 362 to conduct, which in turn renders diode 365 non-conducting, thus producing a pulse as shown at II in waveform 5k to occur across resistor 36|' allowing tubes 361 and 368 to conduct. Tube 361 develops across resistor 313 a negative pulse which again makes tube 315 non-conductive. Since it is assumed that the negative pulse Yapplied to the grid 0f tube 358 is at its minimum of negative value, the current .decrease in resistor 314 due to tube 315 rbeing made non-conducting is less than the current increase in resistor 314 due to tube 358 becoming operative. Hence, a negative pulse is developed across resistor 314 and is coupled to 4lead II5 through coupling tube 384.k Diode 366 faganst vconducts when tube 352 becomes nonconductive. The pulse .from channel I-I inchannel bank J ceases, but since tube 353 is cut-of at this time, nothing happens.

On the next following pulse .from pulse -oscil lator A as shown at S in waveform 5a, a riser occurs in the master step voltage wave generator which causes it to discharge. The occurence of this top riser results in a riser occurring in condenser 3IEJ `in counter D. This riser causes 'the one shot oscillator 3I2 to produce a pulse which discharges all step'voltage wave generators simultaneously, as shown, and also to produce 'a positive pulse across resistor 3II which is coupled to synchronizing pulse generator C via lead |52'. Synchronizing pulse generator C then produces a pulse across resistor 388 in a manner as previously described. The pulse thus developed across resistor 388 and shown in Waveform 5p is coupled to lead I I5 by normally non-conducting coupling tube 385 and appears as shown at S in curve Eq between the pulses from channel I I and I.

The next following pulse from A as shown at I in curve 5a produces a number one riser in the curve 5b of Fig. 5 Which in turn produces a number one riser in curve 5d. The operation of all channels over a complete time period may be similarly described.

Transmittng channel unit Fig. 4 shows, schematically, the circuit details of one channel unit such as might be used in channel banks H, J and K. The operation of this unit is as follows: A step Voltage wave is coupled to the grid of normally non-conducting vacuum tube 508 through a grid limiting resistor 50|. Anode potential is supplied to tube 508 from -i-Ebb over a path including series connected resistors 503 and 502. Tube 508 is biased to become operative on any desired riser in the -applied step voltage wave by means of cathode potentiometer 5 I 0. Each time tube 508 conducts, a charge is stored in condenser 509. This charge leaks off through potentiometer 5I0 during the r time when tube 508 is non-conducting, and develops a voltage of the desired amplitude thereacross. The amplitude of each riser in the applied step voltage wave is made large compared to the cut-off potential of tube 508 in order that tube 508 may be driven from below cut-oli' to zero grid-to-cathode potential on the particular riser which makes tube 508 operative. When the grid-to-cathode voltage of tube 508 reaches zero, it remains at zero for the remainder of the applied step voltage due to a voltage drop being deveoped across reistor 50| due to grid current in tube 508. In other words, as the input wave increases above the value at which the grid-tocathode potential of tube 508 reaches zero, a drop equal to the increase in input voltage is developed across resistor 50| due to grid current from 508. As a result, the anode voltage of tube 508 suddenly drops to a low value when tube 508 conducts and remains at this low value for the remainder of the step voltage Wave.

Pulse modulator vacuum tube 5 I4 is made normally non-conducting by means of a bias voltage developed across resistor 5I3 due to current flowing from -I-Ebb through resistor 505, inductance 506, diode 501 and resistor 5I3 to ground.

When tube 503 becomes conductive, electrons ow into the anode end of condenser 504, thus causing electrons from the other plate of condenser 504 to flow through inductance 506. This results in a negative voltage being developed Y?- across 506 which reduces the anode potential of 'diode 501 below its cathode potential thereby making it non-conductive. When diode 506 becomes non-conductive, the bias on tube |4 is removed and current starts to iiow therein. The amplitude of current flow in 514 is controlled by the modulating signal applied between its grid and cathode by means of audio transformer 5|6 and potentiometer 5|5. Resistor 5|3 is of a value such that tube 5|4 operates as a class A amplier during its conducting period. That is, during the conductive period of tube 5|4, it operates on the linear portion of its grid voltage-1 anode current characteristic curve. Tube 5|4V conducts for a period of time determined by the time determined by the time interval during which diode 501 is cut-off which time interval, in turn, is controlled by the values of condenser 504 and inductance 506; Condenser 504 and inductance 506 maybe considered a series tuned circuit between the anode of tube 508 andresistor 505. When tube 504 starts to conduct, a negative voltage is developed across diode 501 which causes it to cease'conducting and hence oiers a high impedance tov the tuned 'circuit which then tends to oscillate at its natural irequency determined by the Values of condenser 504 and inductance 506. When this oscillation` again rises to a value above the cathode potential of diode 501, it conducts and applies considerable damping across the network thus dissipating .thev energy stored therein. 1

The anode of tube 5|4 is connected, in anelectrically parallel relationship with the anodes of similar tubesin the other channel units in the same bank, to a commonload resistor 5H. Since each channel in the bank is" made operative on a dierent riser in the applied step voltage wave, and since the length of the pulse applied to each modulator tube is short compared to the spacing between the successive risers in thestep voltage wave, a series of non-overlapping, nega` tive, amplitude modulated pulses as shown Aby waveform 006 in Fig. 4 is developed acrossresistor 5H and coupled via lead ||0 or or ||2 to the section of the i PAM converter S assigned to that particular channel.

Resistor 503and condenser 5|| form a com-1 pensation network which maintains the cathode to anode potential, before conduction, constant regardless of the riser upon which tube 508 is made operative. That is, as the cathode 'voltage is increased due to operating on high value risers, the voltage at point P' automatically increases by an amount equal to the increaselin cathode voltage, resulting in the potential between point P and cathode remaining constant. The operation is as follows: On the lower risers, tube 508 carries current for aconsiderably longer period of time than on the top or higher risers. Thus, the average current in tube 508 depends upon the riser upon which tube 508 becomes conducting. When tube 508 conducts on the lower risers, the larger average current flow develops a larger voltage drop across resistor 503 than when tube 500 conducts on the higher risers. Since the cathode bias is lower when tube 508 is made conducting on the lower risers, than when conduct-'1 ing on the higher risers, the value of resistorY 503 may be chosen to be of a Value such that the -A drop there-across decreases in direct proportion` to the increase in cathode voltage, since the average anode current varies in a -linear `fashion With v 18' pass the A. C. components of plate current and thereby provide a D. C. voltage at point P.

Resistor 505 is chosen to provide the desired drop across resistor 5|3 and condenser 5|2 is used as a 'bypass condenser to maintain point M at ground potential for the A. C. components of current in 506.

Operation of receiving system The operation of the receiving multiplex equipment will now be given with particular reference to Figs. 6a, 6b and 8. The same reference nu-v .rnerals` appearing in Figs. 2 and 6a, 6b represent the same circuit elements. The video pulse train from the radio frequency receiver (20| of Fig. 2), as shown in waveform 8a of Fig. 8 is coupled to the grid of a normally conducting cathode output amplifier vacuum tube 404, lead 30| and coupling condenser 40|. circuit of 404 forms the ground return path for D. C. The pulse train developed across resistor 403 by tube 404 is coupled via lead 301 to all channel units which haveV their video input terminals connected in parallel electrically. The

' pulse train developed across cathode resistor 403 is also coupled to a normally no-n-conducting amplifier vacuum tube 400 via coupling condenser 405. Each-synchronizing pulse and the positive portion of each modulated channel pulse causes tube 408 to conduct and as a result develop pulses across anode resistor 409 as shown in curve 8b.

Resistor 401 and condenser 4|3 supply the cuto bias for tube 408, and resistor 400 provides the D. C. return path for the grid. The negative pulses developed across resistor 409 are coupled to the grid of a normally conducting vacuum tube 4|2, via coupling condenser 4I0. Each synchronizing pulse and each fully modulated channel pulse causes tube 4|2 to be cut-off. Since the duration of the synchronizing pulse is made long compared to duration of the channel pulses, tube 4|2 is cut-olf for a longer period of time when the synchronizing pulses are present that when the channel pulses are present on its grid. Re; sistor l4|4 provides the D. C. return path for ther Each time tube 4|2 cuts oi,"

grid of tube 4 l2. condenseri starts to charge up through resistor 4H, resulting in the voltage developed there-across increasing in an exponential manner toward iEbb as a limit. The values of resistor 4|| and condenser 4|0 are chosen to be of :such a Value that the voltagezrise across 4|8 is approximately linear with respect to time for the duration of the synchronizing pulse. At the end of each pulse applied to the grid of tube 4|2, it again becomes conducting and discharges condenser 4|8 resulting in the Voltage developed there-across dropping to a low value. It will readily be seen that the rise in voltage across condenser 4|8 is higher when the synchronizing pulse is present on its grid than when the chan off during the time interval between successive' synchronizing pulses. Bias voltage is supplied to tube 423 by means of cathode resistor 420 and bypass condenser 42|. The waveform 0c of 8 indicates the sawtoothwave developed across condenser 4 I8 and applied to the grid of tube 423.` The horizontal dash-'dot line indicates thepo`v tential above which tube 423 becomes conduct@ Resistor 402 in the grid ing. Inductance 4| 1 -and `resistor 4| 6 in the anode circuit .of tube 423 for a dierentiating circuit across `which is produced a positive pulse at the end of each synchronizing pulse. `That is, at the start of current flow in inductance 4|?|, due to tube 42.3 becoming conducting, .a negative pulse is developed there-across and when tube 423 cuts oi at the end of each pulse, a positive pulse is developed across 4| 1 due vto the eld produced there-in collapsing. Resistor 4|6 critical- 1y Adamps 4|1 to prevent undesired oscillations. The pulses developed across 4|1 are coupled to the grid of a normally non-conducting vacuum tube 424 via coupling condenser 4 I 8.

The pulses coupled to the grid of tube 424 are as shown in curve 8d of Fig. 8. The horizontal dashed line Eco indicates the potential above which tube 424 becomes conducting. Bias is supplied to the grid of tube 424 via cathode resistor 425 and by-pass condenser 426. Resistor 422 in the grid circuit of tube 424 provided the D. C. path to ground lfor the grid. A pulse transformer 421 in the anode circuit of tube 421 is so poled that a positive pulse is applied to the grid of normally non-conducting cathode output vacuum tube 430 each time 424 conducts. The positive pulses as shown in curve 8e developed across resistor 43| by tube 430 are coupled to the discharge tubes in the three substep wave generators via coupling condenser 432 and lead 308. Resistor 433 provides a common D. C. return path for all discharge tubes. The positive pulses developed across resistor 43| are also coupled to the grid of a normally conducting vacuum tube 431 via condenser 434. Condenser 434 and potentiometer 435 in the `grid circuit of tube 431 provide a differentiating network which operates as follows: On each positive pulse across resistor 43| electron current from the grid of tube 431 ows into condenser 434. When the pulse across resistor 43| ceases, the charge stored in condenser 434 leaks oi through potentiometer 435 developing a voltage there-across sufficient to cause tube 431 to become non-conducting. The duration of the non-conducting period of tube 431 is determined by the values of -potenti ometer 435 and condenser 434. The value of impedance offered by potentiometer 435 may be varied by means of the adjustable tap thereon. The result is a positive pulse developed across anode resistor 436. The duration of this pulse is manually adjustable by means of the potentiometer 435 in the grid circuit of tube 431. This positive pulse is coupled to the grid of a normally non-conducting vacuum tube `493 via coupling condenser 439. Cut-off bias for tube 493 is supplied by means of cathode resistor 444 and condenser 445. Resistor 438 provides the D. C. return path for the grid of tube 493. The result is that tube 493 carries current for a period of time determined by the setting of potentiometer 435. When current starts to flow in tube 493, a negative pulse is developed across the inductance 44| and when current ceases a positive pulse is 'developed there-across in a manner similar to that described for tube 423, and inductance 4|1. The occurrence time of the positive pulse is a function of the setting of potentiometer 435. Resistor 440 critically damps the inductance 44|. The pulses developed across inductance 44| are coupled to the grid of a normally non-conducting vacuum tube 45| which is biased below cut-off by means of cathode resistor 441 and condenser 448. Each positive pulse across 44| causes tube 45| to become conducting and produce a negative pulse 20 across its anode resistor 443 as yshown 'in curve 8f. n

'This last negative pulse is coupled to a tuned circuit consisting fof inductance 450 and condenser 449 which is tuned to the frequency of the crystal in the transmitting .multiplex equipment. `Condenser 452 ymust be of low capacity in order to prevent damping across the tuned circuit. The :result is a slightly damped sine wave, shown in curve 8g, is developed across v45S] and applied Yto the grid of a cathode ampliiier vacuum tube 456 which has a cathode resistor common to common tube 451. Tubes 456 and 451 for-m a clipper and limiter stage. That is, on the negative half `cycles of the oscillations applied to the grid of .tube 456, it 'becomes non-conducting. On the positive half cycles of roscillations across 450 ltube 456 conducts and develops a voltage across resistor .456 Vsu-flicient to cause tube 451 to .become non-conducting. The result is a square wave voltage is developed across anode resistor 454 and 'is coupled to resistor 458 in the grid circuit `of tube 46| via condenser 455. Condenser 455 and resistor -458 Aform yan RC diferentiator network which produces alternate positive and negative pulses. That is, on the positive going portion of each cycle vof the sinusoidal voltage developed across resistor 454, a positive pulse is developed across resistor 458, and on the negative .going portion, a negative pulse is developed across resistor 458.

Tube 46|, together with pulse transformer 462, forms `a pulse -oscillator identical to that driven by the crystal oscillator in the transmitting unit. The result is that pulses are produced on the grid of .a master ystep wave .generator charge tube of the same shape `and frequency as those applied to a similar step charge tube in the transmitting equipment. The master step wave generator consisting of vacuum tubes 466, 468, 418 and 41| operates ina manner similar to that described for the master step wave generator in the transmitting unit. Tube 466 charges a storage condenser -461 across which is coupled the anode of a normally non-conducting tube 468, the grid of a cathode output ampliiier 410 and the grid of a one-shot oscillator 41|. Tube 41| is biased by means yof .resistor 414 :and condenser 413 so that a predetermined number 'of pulses (set to be the same as the transmitter unit) cause the voltage across condenser 461 to rise to a value which causes tube 41| to `start to conduct and, due to connections of transformer 412, continue to conduct until condenser -461 is discharged. Tube 468 provides a locking-in arrangement to force the master step wave generator to be in the proper time relation with respect '-to the transmitting equipment. That is, on each received synchronizing pulse, tube 468 becomes conducting and insures the discharge of condenser 461 to occur at this time, thus starting a new cycle of counting of the .master step wave generator. Once the master step wave generator is in the proper time relation with respect to the received synchronizing pulse, it remains so fixed unless the path between the transmitter and receiver is interrupted, after which the received synchronizing pulse again causes it to come into the proper time relation.

That is, the master step wave generator, in the absence of pulses from the synchronizing pulse separator, could start counting on any pulse from the pulse oscillator. But vwhen the synchronizing pulse is applied to tube 468 vcounting `is assured to .start on Ja. -desired pulse. Curve 8h shows the Waveform from the pulse oscillator (note that this waveform is identical to that of a) and curve 8i shows the step voltage wave from the master step wave generator in the receiver (also note curves 8i and 5b are identical) It will be seen that the received synchronizing pulse, curve 0e, occurs at a time following the discharge of the master step voltage wave and preceding the occurrence of the number one riser, that is, after condenser 461 has been discharged by tube 41|. If condenser 401 had a charge at this time due to the master step Wave generator being out of step, condenser 461 would be discharged by tube 468 and the number one riser would occur at the proper time. It should be further noted that there are several cycles of the step voltage wave of curve 8i between successive received synchronizing pulses.

Substep wave generators 208, 209 and 2I0 are identical to the sub-step wave generators in the transmitting unit. Components in 209 and 2|0 similar to those in 208, are given prime and double prime notations.

A Very brief description of substep wave generator will be given here since the operation is the same as that described for the substep wave generators in the transmitter with the exception that the differentiating component in the cathode of the pulse oscillator has been eliminated.

Vacuum tube 43| is a position selector which causes pulse oscillator vacuum tube 484 to produce a pulse at the time of occurrence of a particular riser in the step wave from the master step wave generator applied to the grid of tube 48| through resistor 415. Each pulse from tube 404 causes a charge to be stored in condenser 481. The anode of a normally non-conducting discharge tube 490 is connected to the cathode end of condenser 431 and causes it to be discharged at the time of arrival of the synchronizing .pulse (curve 8e). Vacuum tubes 490, 490' and 490 and 468 are supplied bias by means of grid leak 433 and condenser 432. That is, grid current ows into condenser 432 each time the received synchronizing pulse produces a pulse across resistor 43|. This charge leaks orf during the time interval between successive pulses across resistor 43| developing a bias across 432 sucient to maintain the step discharge tubes non-conducting. A cathode follower vacuum tube 49| couples the step voltage wave developed across condenser 439 to a bank of receiving channel units I-I-I.

The step voltage waves generated by substep wave generators 208, 209 and 2| 0, are as shown in waveforms 87', 8k and Bl respectively. Note that all three step waves are discharged at the end of the received synchronizing pulse.

The step voltage wave generated by substep wave generator 208 is coupled to a bank of receiving channel units H-I by means of cathode output amplifier tube 49|. The step voltage wave generated by substep wave generator 209 is coupled to a bank of receiving channel units J-I by means of cathode output amplifier 49| and the step voltage wave output from substep wave generator 2|0 is coupled to a bank of receiving channel units K-I via cathode output tube `49 I Receiving channel unit All receiving channel units are identical eX- cept for the bias on the position selector tube. The circuit details of a typical receiving channel unit are shown in Fig. '1. The operation of this circuit is as follows: `Vacuum tube A608 vis the 22' channel position selector which allocates the channel unit to its proper position in the step voltage wave applied to the channel in a manner similar to that described for the channel position selector in the transmitting channel unit (tube 508, Fig. 4). It is for this reason that the components 60| to 6I4 of Fig. 7 have been given the same last two reference number designations as the correspondingly located components of Fig. 4. Bias to tube 609 is supplied by means of cathode potentiometer 6I0 and condenser 609. Resistor 603 and condenser 6I I form a compensationnetwork which maintains the D. C. voltage between point P and the cathode constant regardless of the riser upon which tube 608 is made operative. Each time the selector is made operative, due to amplitude of the step wave exceeding the bias on tube 608, a negative pulse of voltage is developed across coil 506 in a manner similar to that described for the transmitting channel unit (coil 506, Fig. 4). This negative pulse causes normally conducting diode E01 to cease conducting by reducing its anode potential to a value lower than that present on its cathode. When diode 6M is conducting, a positive voltage is developed across resistor 6I3 suii'icient to render tube ||4 in-operative. The value of resistor 5i3 is so chosen that when diode 601 is cut-ofi, tube S I4 operates as a class A ampliiier. That is, it operates on the linear portion of its grid-voltage-anode current characteristic curve. Resistor 605 is chosen to provide the desired amplitude of voltage drop across resistor GIS when diode 601 is conducting.

The pulse train from the receiver video output is coupled to the grid of tube @It via lead 301 which is common to similar grids of all receiving channel units. Diode 001 is cut 01T for a period of time equal to the duration of the applied signal pulse. The result is tube Iili is made conducting for a period time equal to the duration cf the applied channel pulses and at a time when a pulse from a particular channel is present on its grid. That is, once each frame, tube @I4 is made conducting when the pulse assigned to the channel is present on its grid. The pulses amplified by tube 5I4 are coupled to a low pass filter 0I1 via coupling condenser SIE. Anode resistor SI5 is of a value such that the lter is driven from its proper impedance. The filter is terminated by a potentiometer 6I8 which has an impedance equal to the terminating impedance required by the filter. A tap on potentiometer @I8 provides a means or" manually adjusting the amplitude of the audio signal coupled to the audio ampliiier which consists of two triode type tubes 62| and 20 operating as class A amplifiers. The grid of a voltage amplifier tube 82| is directly connected to the tap on potentiometer SIB as shown in Fig. '1 and bias is supplied by means of resistor 523 and condenser 622. The ampliiied audio signal developed across anode resistor SI2 is coupled to the grid of output ampliiier tube via coupling condenser 620. Resistor S24 provides the ground return path for the grid of tube 32e. Resistor 625 is anv un-bypassed cathode den generation resistor in the cathode circuit of tube 62B. Transformer 621 in the anode circuit or tube 026 is an output type transformer which matches the impedance of tube 026 to the output line impedance. The Aoperation of the low pass lter SI1 is as fol1ows:' The signal developed across SI5 when tube 6I4 becomes conducting is a pulse, the amplitude of which is varied in ac'- cordance with the modulating signal applied to 

